This invention relates to a stress-strain measuring apparatus and methods and more particularly to such apparatus and methods which apply a constant magnetic field to a workpiece during the application and measurement of stress.
Accurate knowledge of the stresses and strains associated with the service loading of structural components is critical to the design of safe, efficient hardware. Common practice is to estimate expected loading conditions through the use of analytical methods and subsequently, to verify these estimates through model testing or in-service monitoring of operating equipment. Experimental measurement of stresses and strains involves the use of strain gauges, such as electrical resistivity gauges designed to respond to changes in the strain, which must be mounted on the hardware of concern at the specific locations of interest. The hardware is then loaded and measurements of strain made at each gauge. The strain measurements are converted to applied stress values using Young's Modulus for the structural material involved to yield an experimental stress analysis. This approach to stress analysis, while often used successfully, has several significant limitations. Specifically, strains can only be measured at the predetermined gauge locations, expensive and complex slip ring or telemetering equipment is required to monitor strains in rotating equipment and strain gauges alone cannot be used to detect residual stresses which may have developed during fabrication.
The use of eddy current techniques to characterize the properties of materials is based on the relationship between material structure and electromagnetic properties. More specifically, eddy current testing relies on electromagnetic interaction between a coil driven by an alternating electrical current and the material under test. In order to measure applied or residual stresses with a conventional eddy current approach, the application of stress must change either the conductivity or permeability of the material such that a detectable change in the test coil impedance occurs. For ferromagnetic materials, it is well known that the application of stress will alter both the electric and magnetic properties. This phenomenon permits the use of eddy current techniques to measure applied and residual stress in structural steels.